Segmented assemblies for reciprocating compression and pump equipment are well-known in the art. Known segmented sealing ring assemblies often have multiple parts, for example a cover ring and a sealing ring, that will tend to wear at different rates over time due to the differences in pressure being exerted on the parts. In reciprocating piston systems, the rate of wear of the seal assembly rings is proportional to the contact stress the respective ring experiences when brought into contact with the sliding surface (for example the inside of the cylinder shaft). The higher the contact stress, the higher the consequent rate of wear. This contact stress is proportional to the pressure being sealed or, more accurately, the pressure differential across the seal assembly. The larger this pressure differential, the greater the contact stress acting on the seal assembly.
These pressure differentials cause the sealing ring to tend to wear more quickly than the cover ring due to the way that pressure acts on each of the rings over the life of the seal assembly. The tendency of the sealing ring to wear at a higher rate than the cover ring is referred to in the art as “preferential wear.” Over time, preferential wear leads to a gap opening up between the sealing ring and the cover ring, as further discussed herein. This problem was recognized in a prior art technical publication entitled “Influence of piston ring design on the capacity of dry-running hydrogen compressor” (Dr. Norbert Feistel; Published by Burckhardt Compression AG of Winterthur, Switzerland). One such prior art device is disclosed in U.S. Pat. No. 4,185,842, which teaches a seal assembly comprising a sealing ring and a cover ring that is L-shaped in cross-section.
The gap in the cover ring would allow leakage similar to a butt joint ring, except the sealing ring covers this gap in the cover ring. Similarly, the gap in the sealing ring would allow leakage similar to a butt joint ring, except the cover ring covers this gap in the sealing ring. Initially, therefore, as a set of rings, the seal assembly is capable of sealing the entire circumference of the shaft or piston. The pressure differential acting on the seal assembly is relatively uniform when new, with the exception of the gap in the cover ring where the sealing ring is exposed and performs all of the sealing by itself. It is at this location that the preferential wear begins to occur. The sealing ring is exposed to the full pressure differential at this location, so the cover ring does none of the sealing at this location. Because the sealing ring has a high differential pressure across it at this location and the cover ring does not, the contact stress acting on the sealing ring is much higher than the contact stress acting on the cover ring. Therefore, the wear rate of the sealing ring is much higher than the wear rate of the cover ring. As the sealing ring begins to wear, a circumferential gap opens up between the cover ring and the sealing ring. This has the effect of increasing the radial location where the sealing ring alone is providing all of the sealing. As the radial length of this circumferential gap increases, the cover ring eventually no longer provides any sealing, the differential pressure across the cover ring and the contact stress acting on the cover ring declines, and the wear rate of the cover ring is reduced. The circumferential gap that has opened between the cover ring and the sealing ring continues to increase in size until it has circumnavigated the entire seal. The sealing ring thus becomes similar to a butt joint ring, allowing gas to flow through the gap in the sealing ring.
Other relevant prior art references include U.S. Patent Application Publication No. 2003/0006562 and U.S. Pat. No. 6,322,080. U.S. Patent Application Publication No. 2003/0006562 discloses a seal assembly which has a concentric tongue and groove arrangement which mechanically couples the two rings together to make up the seal assembly. With this arrangement, the two rings are coupled together so that wear on the sealing ring tends to produce a force on the cover ring in the direction of the pressure. In effect, the sealing ring ‘pulls’ the cover ring into the shaft or cylinder against which the sealing ring is running. By doing so, the cover ring presses against the moving shaft or cylinder, increasing its contact stress against the sliding surface and decreasing the contact stress of the sealing ring against the sliding surface. By coupling the two rings together in this manner, uneven ring wear can be minimized. However, the presence of the tongue and groove arrangement in this design produces shearing stress concentrations on the tongue portion of the sealing ring, and can result in failure of the seal assembly. U.S. Pat. No. 6,322,080 discloses a sealing ring having a radial wall thickness that decreases towards the area of the sealing ring gap. As opposed to the present invention, only the sealing ring of this device makes contact with the sliding surface (La, the moving shaft of the piston) and not the cover ring.
There is a need for a seal assembly design that compensates for the different wear rates of the cover ring and seal ring in a manner that does not introduce the other potential premature failure modes associated with the prior art.